Apparatus and method for encoding and decoding a picture using picture boundary handling

ABSTRACT

The present invention concerns an apparatus configured to partition a picture into leaf blocks using recursive multi-tree partitioning, block-based encode the picture into a data stream using the partitioning of the picture into the leaf blocks, wherein the apparatus is configured to, in partitioning the picture into the leaf blocks, for a predetermined block which extends beyond a boundary of the picture, reduce an available set of split modes depending on a position at which the boundary of the picture crosses the predetermined block in order to obtain a reduced set of one or more split modes, wherein the apparatus is configured to signal a selected split mode in the data stream.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of copending InternationalApplication No. PCT/EP2019/057170, filed Mar. 21, 2019, which isincorporated herein by reference in its entirety, and additionallyclaims priority from European Application No. 18165218.1, filed Mar. 29,2018, which is also incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

The present invention is concerned with picture and video coding.Embodiments of the present invention are concerned with a particular wayof partitioning input picture and video data of a specific size intosmaller entities. In particular, embodiments of the present inventionare concerned with picture boundary handling in recursive picture signalpartitioning.

In modern video coding applications, the input video signal of aspecific size is partitioned into smaller chunks [1]. This partitioningconsists of multiple structures with specific information anddescription associated with each level. In the state of the art videocodec H.265/HEVC [1], the most important subdivision is the subdivisioninto macroblocks. Those macroblocks, or coding tree units, are quadraticstructures of a predefined size spanning a fixed grid over the codedpicture. All other partitioning levels are defined in terms of thisrigid subdivision. E.g., the more coarse high level partitioning intoslices and tiles is defined in terms of the included CTUs.

In H.265/HEVC [1], starting at each CTU, a quad-tree structure issignaled providing means of recursively partitioning a rigid CTU intoflexible sub-structures. At each level, in a defined parameter scope, itis inferred or signaled a block that should be split into foursub-blocks or coded at the specified level. This process is repeatedrecursively until no further split is possible as defined by currenthigh-level parameters or a flag is read indicating that no further splitshould be performed at the current level. In this case, signaling of acoding unit and its sub-structures will be read.

The signaling of a quad-tree split can sometimes be omitted and inferredas either true or false. Most notably, due the rigid nature, CTUs arenot necessarily aligned with video picture boundaries. In this case, ifa CTU or a sub-block in the quad-tree partitioning structure is notfully contained in the picture boundaries, a split flag will be inferredas positive without explicit signaling. If, after the split, a sub-blocklies completely outside of the video picture boundaries, no furthersignaling for this block will be read.

In the development of a future video standard with capabilities beyondH.265/HEVC [1], the quad-tree recursive splitting structure is extendedby different split configurations [2]. If a block is not split into fourquadratic sub-parts, a binary split is signaled indicating that thisblock should be split into two rectangular blocks using a split ratio of1/2. The signaling also includes the information if the split should beapplied horizontally or vertically. Such rectangular blocks can befurther recursively split into smaller quadratic or rectangular blocksusing the binary tree split syntax. This enhanced split-tree is calledQTBT [2]. QTBT handles picture boundaries using the implicit quad-split,exactly as in the H.265/HEVC standard [1]. In QTBT, if a block exceedsthe picture boundaries, a quad-split will be inferred and no signalingwill be read.

In [5], a novel partitioning method is described, Generalized BinarySplitting, which might not contain a quad-split but might also be usedtogether with a quad split. In this method the binary split can besignaled to perform the split with a split ratio other than 1/2. Thesplits might also be signaled using a syntax relative to the previoussplit.

However, the conventional partitioning methods are rather rigid in thehandling of picture boundaries. As explained above, conventionalpartitioning methods may use an implicit quad split at pictureboundaries. That is, the known technology uses a fully implicit splitderivation at a picture boundary. Such implicit signaling may enable agood signaling efficiency. However, this may only provide for a lowflexibility.

Thus, it is an object of the present invention to enhance existingpartitioning methods to be more flexible in picture boundary handlingand, at the same time, to reduce the bit budget for signaling thepicture boundary handling.

SUMMARY

An embodiment may have an apparatus for encoding a picture, configuredto partition the picture into leaf blocks using recursive multi-treepartitioning, block-based encode the picture into a data stream usingthe partitioning of the picture into the leaf blocks, wherein theapparatus is configured to, in partitioning the picture into the leafblocks, for a predetermined block that corresponds to a predeterminedtree level of the multi-tree partitioning and which extends beyond aboundary of the picture, reduce an available set of split modes forsplitting the predetermined block depending on a position at which theboundary of the picture crosses the predetermined block in order toobtain a reduced set of one or more split modes, wherein, if acardinality of the reduced set is one, the apparatus is configured toapply the split mode of the reduced set for splitting the predeterminedblock, and if a cardinality of the reduced set is greater than one, theapparatus is configured to select one of the split modes of the reducedset and to apply the selected one of the split modes for splitting thepredetermined block and to signal the selection in the data stream.

Another embodiment may have an apparatus for decoding a picture,configured to partition the picture into leaf blocks using recursivemulti-tree partitioning, block-based decode the picture from a datastream using the partitioning of the picture into leaf blocks, whereinthe apparatus is configured to, in partitioning the picture into theleaf blocks, for a predetermined block that corresponds to apredetermined tree level of the multi-tree partitioning and whichextends beyond a boundary of the picture, reduce an available set ofsplit modes for splitting the predetermined block depending on aposition at which the picture boundary crosses the predetermined blockto obtain a reduced set of one or more split modes, wherein if acardinality of the reduced set is one, the apparatus is configured toapply the split mode of the reduced set for splitting the predeterminedblock, and if a cardinality of the reduced set is greater than one, theapparatus is configured to select one of the split modes of the reducedset and to apply the selected one of the split modes for splitting thepredetermined block according to a signalization in the data stream.

According to another embodiment, a method for encoding a picture mayhave the steps of: partitioning the picture into leaf blocks usingrecursive multi-tree partitioning, block-based encoding the picture intoa data stream using the partitioning of the picture into the leafblocks, wherein the method further comprises the steps of, inpartitioning the picture into the leaf blocks, for a predetermined blockthat corresponds to a predetermined tree level of the multi-treepartitioning and which extends beyond a boundary of the picture,reducing an available set of split modes for splitting the predeterminedblock depending on a position at which the picture boundary crosses thepredetermined block in order to obtain a reduced set of one or moresplit modes, wherein if a cardinality of the reduced set is one, themethod comprises a step of applying the split mode of the reduced setfor splitting the predetermined block, and if a cardinality of thereduced set is greater than one, the method comprising a step ofselecting one of the split modes of the reduced set and applying theselected one of the split modes for splitting the predetermined blockand signaling the selection in the data stream.

According to still another embodiment, a method for decoding a picturemay have the steps of: partitioning the picture into leaf blocks usingrecursive multi-tree partitioning, block-based decoding the picture froma data stream using the partitioning of the picture into leaf blocks,wherein the method further comprises the steps of, in partitioning thepicture into the leaf blocks, for a predetermined block, whichcorresponds to a predetermined tree level of the multi-tree partitioningand which extends beyond a boundary of the picture, reducing anavailable set of split modes for splitting the predetermined blockdepending on a position at which the picture boundary crosses thepredetermined block in order to obtain a reduced set of one or moresplit modes, wherein if a cardinality of the reduced set is one, themethod comprises a further step of applying the split mode of thereduced set for splitting the predetermined block, and if a cardinalityof the reduced set is greater than one, the method comprising thefurther step of selecting one of the split modes of the reduced set andapplying the selected one of the split modes for splitting thepredetermined block according to a signalization in the data stream.

Another embodiment may have a non-transitory digital storage mediumhaving stored thereon a computer program for performing a method forencoding a picture, the method comprising the steps of partitioning thepicture into leaf blocks using recursive multi-tree partitioning,block-based encoding the picture into a data stream using thepartitioning of the picture into the leaf blocks, wherein the methodfurther comprises the steps of, in partitioning the picture into theleaf blocks, for a predetermined block that corresponds to apredetermined tree level of the multi-tree partitioning and whichextends beyond a boundary of the picture, reducing an available set ofsplit modes for splitting the predetermined block depending on aposition at which the picture boundary crosses the predetermined blockin order to obtain a reduced set of one or more split modes, wherein ifa cardinality of the reduced set is one, the method comprises a step ofapplying the split mode of the reduced set for splitting thepredetermined block, and if a cardinality of the reduced set is greaterthan one, the method comprising a step of selecting one of the splitmodes of the reduced set and applying the selected one of the splitmodes for splitting the predetermined block and signaling the selectionin the data stream, when said computer program is run by a computer.

Another embodiment may have a non-transitory digital storage mediumhaving stored thereon a computer program for performing a method fordecoding a picture, the method comprising the steps of partitioning thepicture into leaf blocks using recursive multi-tree partitioning,block-based decoding the picture from a data stream using thepartitioning of the picture into leaf blocks, wherein the method furthercomprises the steps of, in partitioning the picture into the leafblocks, for a predetermined block, which corresponds to a predeterminedtree level of the multi-tree partitioning and which extends beyond aboundary of the picture, reducing an available set of split modes forsplitting the predetermined block depending on a position at which thepicture boundary crosses the predetermined block in order to obtain areduced set of one or more split modes, wherein if a cardinality of thereduced set is one, the method comprises a further step of applying thesplit mode of the reduced set for splitting the predetermined block, andif a cardinality of the reduced set is greater than one, the methodcomprising the further step of selecting one of the split modes of thereduced set and applying the selected one of the split modes forsplitting the predetermined block according to a signalization in thedata stream, when said computer program is run by a computer.

Still another embodiment may have a data stream obtained by the aboveinventive method of encoding.

Another embodiment may have a data stream obtained by the aboveinventive method of decoding.

A first aspect concerns an apparatus for encoding a picture. Theapparatus is configured to partition the picture into leaf blocks usinga recursive multi-tree partitioning. That is, the apparatus isconfigured to partition the picture into smaller chunks using saidrecursive multi-tree partitioning. Said smaller chunks may be blocks ofa certain size. By stepping through the multi-tree, the partitioning ofthe picture may start from a tree-root block at a first level and it mayend up at a leaf block, which is the last block of the multi-tree andthus the smallest entity of the partitioning. Between the tree-rootblock and the partitioned leaf block, the apparatus may step-by-step gofrom one partitioning tree level to one or more subsequent partitioningtree levels, wherein at each tree level the current block at theparticular tree level is partitioned into two or more smaller blocks.For example in HEVC, the tree-root blocks may be so-called macroblocksor Coding Tree Units, and the leaf blocks may be so-called subblocks orCoding Unit. Accordingly, a CTU may be partitioned into one or more CUs,wherein a leaf block or any block at a tree level between the tree-rootblock and the leaf block may be referred to as a subblock. Furthermore,in the present disclosure, a tree-root block, or a leaf block, or anyblock at a tree level between the tree-root block and the leaf block maybe referred to as a predetermined block. Accordingly, a predeterminedblock may correspond to a predetermined tree level of the multi-treepartitioning. The apparatus may exploit a block-based coding scheme,i.e. the apparatus is configured to block-based encode the picture intoa data stream by using the partitioning of the picture into the leafblocks. The partitioning may also be referred to as splitting. Thepartitioning of the tree-root blocks into the smaller leaf blocks mayuse a certain splitting scheme for splitting blocks into smallersubblocks. These splitting schemes may also be referred to as splitmodes which may vary at each tree level. Furthermore, the predeterminedblocks may comprise a predetermined size. When arranging thepredetermined blocks as a grid over the picture, it may happen that someof the predetermined blocks, due to its size, may extend over a pictureboundary. For example, a first portion of a predetermined block may belocated inside the picture while a second portion of said predeterminedblock may be located outside the picture. Accordingly, the pictureboundary may cross the predetermined block. Generally, the pictureboundary may cross predetermined blocks at different positions. Forexample, the picture boundary may cross a predetermined blockhorizontally, or vertically, or both wherein the picture corner will becontained in the predetermined block. For handling these situations, thepresent invention provides the following solution. While partitioningthe picture into the leaf blocks, the apparatus is configured topartition a predetermined block that corresponds to a predetermined treelevel of the multi-tree partitioning and which extends beyond a boundaryof the picture, by using a reduced set of split modes compared to fullyimplicit split derivation as used by the known technology. According tothe invention, the apparatus is configured to reduce an available set ofsplit modes for splitting the predetermined block depending on the abovementioned position at which the picture boundary crosses saidpredetermined block. Thus, the apparatus obtains a reduced set of one ormore split modes. The reduced set may comprise a cardinality whichindicates the number of split modes being available in the reduced setof split modes. Accordingly, if a cardinality of the reduced set is one,i.e. if the reduced set comprises only one split mode, the apparatus isconfigured to apply this split mode of the reduced set for splitting thepredetermined block. If a cardinality of the reduced set is greater thanone, i.e. if the reduced set comprises two or more different splitmodes, the apparatus is configured to select one of these split modes ofthe reduced set and to apply the selected one of the split modes forsplitting the predetermined block, wherein the apparatus signals itsrespective selection in the data stream. In other words, the inventiveapparatus may pre-select a reduced set of split modes for splitting acurrent predetermined block. Said pre-selection may depend on thecurrent position of the block relative to the picture boundary. Inresult, the inventive apparatus may only have to choose a suitable splitmode from a pre-selected reduced set of split modes. This reduces thebit budget for signaling the selected split mode in the data streambecause, no information about splitting itself needs to be transferred,and if the reduced split set comprises more than one split mode, onlythe remaining uncertainty may have to be signaled, e.g. by one bin.

A second aspect concerns an apparatus for decoding a picture. Theapparatus is configured to partition the picture into leaf blocks usingrecursive multi-tree partitioning. The apparatus is further configuredto block-based decode the picture from a data stream using thepartitioning of the picture into leaf blocks. The apparatus is furtherconfigured to, in partitioning the picture into the leaf blocks, for apredetermined block that corresponds to a predetermined tree level ofthe multi-tree partitioning and that extends beyond a boundary of thepicture, reduce an available set of split modes for splitting thepredetermined block depending on a position at which the boundarycrosses the predetermined block to obtain a reduced set of one or moresplit modes. If a cardinality of the reduced set is one, the apparatusis configured to apply the split mode of the reduced set for splittingthe predetermined block, and if a cardinality of the reduced set isgreater than one, the apparatus is configured to select one of the splitmodes of the reduced set and to apply the selected one of the splitmodes for splitting the predetermined block according to a signalizationin the data stream. As to the advantages of said apparatus for decodinga picture it is referred to the passage above describing the advantagesof the apparatus for encoding a picture.

A third aspect concerns a method for encoding a picture, the methodcomprising a step of partitioning the picture into leaf blocks usingrecursive multi-tree partitioning. The method further comprises a stepof block-based encoding the picture into a data stream using thepartitioning of the picture into the leaf blocks. The method furthercomprises the steps of, in partitioning the picture into the leafblocks, for a predetermined block that corresponds to a predeterminedtree level of the multi-tree partitioning and which extends beyond aboundary of the picture, reducing an available set of split modes forsplitting the predetermined block depending on a position at which theboundary crosses the predetermined block to obtain a reduced set of oneor more split modes. If a cardinality of the reduced set is one, themethod comprises a step of applying the split mode of the reduced setfor splitting the predetermined block, and if a cardinality of thereduced set is greater than one, the method comprising a step ofselecting one of the split modes of the reduced set and applying theselected one of the split modes for splitting the predetermined blockand signaling the selection in the data stream. As to the advantages ofsaid method for encoding a picture it is referred to the passage abovedescribing the advantages of the apparatus for encoding a picture.

A fourth aspect concerns a method for decoding a picture, the methodcomprising a step of partitioning the picture into leaf blocks usingrecursive multi-tree partitioning. The method further comprises a stepof block-based decoding the picture from a data stream using thepartitioning of the picture into leaf blocks. The method furthercomprises the steps of, in partitioning the picture into the leafblocks, for a predetermined block that corresponds to a predeterminedtree level of the multi-tree partitioning and which extends beyond aboundary of the picture, reducing an available set of split modes forsplitting the predetermined block depending on a position at which theboundary crosses the predetermined block to obtain a reduced set of oneor more split modes. If a cardinality of the reduced set is one, themethod comprises a further step of applying the split mode of thereduced set for splitting the predetermined block, and if a cardinalityof the reduced set is greater than one, the method comprising thefurther step of selecting one of the split modes of the reduced set andapplying the selected one of the split modes for splitting thepredetermined block according to a signalization in the data stream. Asto the advantages of said method for decoding a picture it is referredto the passage above describing the advantages of the apparatus forencoding a picture.

According to a fifth aspect, computer programs are provided, whereineach of the computer programs is configured to implement theabove-described method when being executed on a computer or signalprocessor, so that the above-described method is implemented by one ofthe computer programs.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present application will be exemplarily describedbelow with respect to the figures, in which:

FIG. 1 shows a block diagram of an apparatus for predictively coding apicture as an example for an encoder where an intra prediction conceptaccording to embodiments of the present application could beimplemented;

FIG. 2 shows a block diagram of an apparatus for predictively decoding apicture, which fits to the apparatus of FIG. 1, as an example fordecoder where an intra prediction concept according to embodiments ofthe present application could be implemented;

FIG. 3 shows a schematic diagram illustrating an example for arelationship between the prediction residual signal, the predictionsignal and the reconstructed signal so as to illustrate possibilities ofsetting subdivisions for coding mode selection, transform selection andtransform performance, respectively;

FIG. 4A shows an embodiment of an encoder according to the presentconcept;

FIG. 4B shows an embodiment of a decoder according to the presentconcept;

FIG. 5A shows a fully unrestricted decision tree comprising a full setof available split modes;

FIG. 5B shows a restricted decision tree comprising a reduced set ofsplit modes according to the present concept;

FIG. 5C shows a further fully unrestricted decision tree comprising afull set of available split modes;

FIG. 5D shows a restricted decision tree comprising a reduced set ofsplit modes according to the present concept;

FIG. 6 shows a schematic illustration of different cases how blocks mayextend beyond picture boundaries;

FIG. 7A shows an example of a quad split mode;

FIG. 7B shows an example of a vertical bi-split mode;

FIG. 7C shows an example of a horizontal bi-split mode;

FIG. 7D shows an example of a vertical ternary split mode;

FIG. 7E shows an example of a horizontal ternary split mode;

FIG. 8A shows an example of splitting a macroblock into leaf blocksusing quad splits only;

FIG. 8B shows an example of splitting a macroblock into leaf blocksusing quad splits, vertical bi-splits and horizontal bi-splits selectedfrom a reduced set of split modes;

FIG. 9 shows a block diagram of a method for encoding a pictureaccording to the present concept; and

FIG. 10 shows a block diagram of a method for decoding a pictureaccording to the present concept.

DETAILED DESCRIPTION OF THE INVENTION

Equal or equivalent elements or elements with equal or equivalentfunctionality are denoted in the following description by equal orequivalent reference numerals.

The following description of the Figures may only describe some ofmultiple illustrative and non-limiting examples and embodiments of theherein described concept. CTUs may be described as non-limiting examplesof macroblocks, while CUs may be described as non-limiting examples ofsubblocks. Furthermore, a picture may be composed of e.g. multipletiles, each representing an image themselves. In HEVC [1], such tiling,e.g. for omni-directional video, is used and handled by the high levelconcept of picture tiles. Such tiling is available at CTU resolution. Incase such a restriction is loosened, a “coded picture” might consist ofmultiple tiles, each with its own boundaries and with its own content.In that sense, a coded picture might contain more picture boundaries,with each tile constituting its own “picture data” with its own “picturedata” boundaries, for which the same rules as described in thisinvention might apply. Accordingly, if the term “picture boundary” isused herein, this may also include a “current picture data boundary”belonging to, e.g. one or more tiles.

The following description of the figures starts with a presentation of adescription of an encoder and a decoder of a block-based predictivecodec for coding pictures of a video in order to form an example for acoding framework into which embodiments may be built in. The encoder anddecoder are described with respect to FIGS. 1 to 3. Thereinafter thedescription of embodiments of the concept of the present application ispresented along with a description as to how such concepts could bebuilt into the encoder and decoder of FIGS. 1 and 2, respectively,although the embodiments described with the subsequent FIG. 4 andfollowing, may also be used to form encoders and decoders not operatingaccording to the coding framework underlying the encoder and decoder ofFIGS. 1 and 2.

FIG. 1 shows an apparatus for predictively coding a picture 12 into adata stream 14 exemplarily using transform-based residual coding. Theapparatus for encoding, or encoder, is indicated using reference sign10. FIG. 2 shows a corresponding apparatus for decoding, or a decoder20, that may be an apparatus configured to predictively decode thepicture 12′ from the data stream 14 also using transform-based residualdecoding, wherein the apostrophe has been used to indicate that thepicture 12′ as reconstructed by the decoder 20 may deviate from picture12 originally encoded by apparatus 10 in terms of coding loss introducedby a quantization of the prediction residual signal. FIG. 1 and FIG. 2exemplarily use transform based prediction residual coding, althoughembodiments of the present application are not restricted to this kindof prediction residual coding. This is true for other details describedwith respect to FIGS. 1 and 2, too, as will be outlined hereinafter.

The encoder 10 may be configured to subject the prediction residualsignal to spatial-to-spectral transformation and to encode theprediction residual signal, thus obtained, into the data stream 14.Likewise, the decoder 20 may be configured to decode the predictionresidual signal from the data stream 14 and subject the predictionresidual signal thus obtained to spectral-to-spatial transformation.

Internally, the encoder 10 may comprise a prediction residual signalformer 22 which generates a prediction residual 24 so as to measure adeviation of a prediction signal 26 from the original signal, i.e. fromthe picture 12. The prediction residual signal former 22 may, forinstance, be a subtractor which subtracts the prediction signal from theoriginal signal, i.e. from the picture 12. The encoder 10 then furthercomprises a transformer 28 which subjects the prediction residual signal24 to a spatial-to-spectral transformation to obtain a spectral-domainprediction residual signal 24′ which is then subject to quantization bya quantizer 32, also comprised by the encoder 10. The thus quantizedprediction residual signal 24″ is coded into bitstream 14. To this end,encoder 10 may optionally comprise an entropy coder 34 which entropycodes the prediction residual signal as transformed and quantized intodata stream 14. The prediction residual 26 is generated by a predictionstage 36 of encoder 10 on the basis of the prediction residual signal24″ encoded into, and decodable from, data stream 14. To this end, theprediction stage 36 may internally, as is shown in FIG. 1, comprise adequantizer 38 which dequantizes prediction residual signal 24″ so as togain spectral-domain prediction residual signal 24′″, which correspondsto signal 24′ except for quantization loss, followed by an inversetransformer 40 which subjects the latter prediction residual signal 24′″to an inverse transformation, i.e. a spectral-to-spatial transformation,to obtain prediction residual signal 24″″, which corresponds to theoriginal prediction residual signal 24 except for quantization loss. Acombiner 42 of the prediction stage 36 then recombines, such as byaddition, the prediction signal 26 and the prediction residual signal24″″ so as to obtain a reconstructed signal 46, i.e. a reconstruction ofthe original signal 12. Reconstructed signal 46 may correspond to signal12′. A prediction module 44 of prediction stage 36 then generates theprediction signal 26 on the basis of signal 46 by using, for instance,spatial prediction, i.e. intra prediction, and/or temporal prediction,i.e. inter prediction.

Likewise, decoder 20, as shown in FIG. 2, may be internally composed ofcomponents corresponding to, and interconnected in a mannercorresponding to, prediction stage 36. In particular, entropy decoder 50of decoder 20 may entropy decode the quantized spectral-domainprediction residual signal 24″ from the data stream, whereupondequantizer 52, inverse transformer 54, combiner 56 and predictionmodule 58, interconnected and cooperating in the manner described abovewith respect to the modules of prediction stage 36, recover thereconstructed signal on the basis of prediction residual signal 24″ sothat, as shown in FIG. 2, the output of combiner 56 results in thereconstructed signal, namely picture 12′.

Although not specifically described above, it is readily clear that theencoder 10 may set some coding parameters including, for instance,prediction modes, motion parameters and the like, according to someoptimization scheme such as, for instance, in a manner optimizing somerate and distortion related criterion, i.e. coding cost. For example,encoder 10 and decoder 20 and the corresponding modules 44, 58,respectively, may support different prediction modes such asintra-coding modes and inter-coding modes. The granularity at whichencoder and decoder switch between these prediction mode types maycorrespond to a subdivision of picture 12 and 12′, respectively, intocoding segments or coding blocks. In units of these coding segments, forinstance, the picture may be subdivided into blocks being intra-codedand blocks being inter-coded. Intra-coded blocks are predicted on thebasis of a spatial, already coded/decoded neighborhood of the respectiveblock as is outlined in more detail below. Several intra-coding modesmay exist and be selected for a respective intra-coded segment includingdirectional or angular intra-coding modes according to which therespective segment is filled by extrapolating the sample values of theneighborhood along a certain direction which is specific for therespective directional intra-coding mode, into the respectiveintra-coded segment. The intra-coding modes may, for instance, alsocomprise one or more further modes such as a DC coding mode, accordingto which the prediction for the respective intra-coded block assigns aDC value to all samples within the respective intra-coded segment,and/or a planar intra-coding mode according to which the prediction ofthe respective block is approximated or determined to be a spatialdistribution of sample values described by a two-dimensional linearfunction over the sample positions of the respective intra-coded blockwith driving tilt and offset of the plane defined by the two-dimensionallinear function on the basis of the neighboring samples. Comparedthereto, inter-coded blocks may be predicted, for instance, temporally.For inter-coded blocks, motion vectors may be signaled within the datastream, the motion vectors indicating the spatial displacement of theportion of a previously coded picture of the video to which picture 12belongs, at which the previously coded/decoded picture is sampled inorder to obtain the prediction signal for the respective inter-codedblock. This means, in addition to the residual signal coding comprisedby data stream 14, such as the entropy-coded transform coefficientlevels representing the quantized spectral-domain prediction residualsignal 24″, data stream 14 may have encoded thereinto coding modeparameters for assigning the coding modes to the various blocks,prediction parameters for some of the blocks, such as motion parametersfor inter-coded segments, and optional further parameters such asparameters for controlling and signaling the subdivision of picture 12and 12′, respectively, into the segments. The decoder 20 uses theseparameters to subdivide the picture in the same manner as the encoderdid, to assign the same prediction modes to the segments, and to performthe same prediction to result in the same prediction signal.

FIG. 3 illustrates the relationship between the reconstructed signal,i.e. the reconstructed picture 12′, on the one hand, and the combinationof the prediction residual signal 24″″ as signaled in the data stream,and the prediction signal 26, on the other hand. As already denotedabove, the combination may be an addition. The prediction signal 26 isillustrated in FIG. 3 as a subdivision of the picture area intointra-coded blocks which are illustratively indicated using hatching,and inter-coded blocks which are illustratively indicated not-hatched.The subdivision may be any subdivision, such as a regular subdivision ofthe picture area into rows and columns of blocks, or a multi-treesubdivision of picture 12 into leaf blocks of varying size, such as aquadtree subdivision or the like, into blocks, wherein a mixture thereofis illustrated in FIG. 3 where the picture area is first subdivided intorows and columns of tree-root blocks which are then further subdividedin accordance with a recursive multi-tree subdivisioning. Again, datastream 14 may have an intra-coding mode coded thereinto for intra-codedblocks 80, which assigns one of several supported intra-coding modes tothe respective intra-coded block 80. Further details are describedbelow. For inter-coded blocks 82, the data stream 14 may have one ormore motion parameters coded thereinto. Generally speaking, inter-codedblocks 82 are not restricted to being temporally coded. Alternatively,inter-coded blocks 82 may be any block predicted from previously codedportions beyond the current picture 12 itself, such as previously codedpictures of a video to which picture 12 belongs, or picture of anotherview or an hierarchically lower layer in the case of encoder 10 anddecoder 20 being scalable encoders and decoders, respectively. Theprediction residual signal 24″″ in FIG. 3 is also illustrated as asubdivision of the picture area into blocks 84. These blocks might becalled transform blocks in order to distinguish same from the codingblocks 80 and 82. In effect, FIG. 3 illustrates that encoder 10 anddecoder 20 may use two different subdivisions of picture 12 and picture12′, respectively, into blocks, namely one subdivisioning into codingblocks 80 and 82, respectively, and another subdivision into transformblocks 84. Both sub-divisions might be the same, i.e. each coding block80 and 82, may concurrently form a transform block 84, but FIG. 3illustrates the case where, for instance, a subdivision into transformblocks 84 forms an extension of the subdivision into coding blocks 80,82 so that any border between two blocks of blocks 80 and 82 overlays aborder between two blocks 84, or alternatively speaking each block 80,82 either coincides with one of the transform blocks 84 or coincideswith a cluster of transform blocks 84. However, the subdivisions mayalso be determined or selected independent from each other so thattransform blocks 84 could alternatively cross block borders betweenblocks 80, 82. As far as the subdivision into transform blocks 84 isconcerned, similar statements are thus true as those brought forwardwith respect to the subdivision into blocks 80, 82, i.e. the blocks 84may be the result of a regular subdivision of a picture area intoblocks, arranged in rows and columns, the result of a recursivemulti-tree subdivisioning of the picture area, or a combination thereofor any other sort of blockation. Just as an aside, it is noted thatblocks 80, 82 and 84 are not restricted to being of quadratic,rectangular or any other shape.

FIG. 3 illustrates that the combination of the prediction signal 26 andthe prediction residual signal 24″″ directly results in thereconstructed signal 12′. However, it should be noted that more than oneprediction signal 26 may be combined with the prediction residual signal24″″ to result into picture 12′ in accordance with alternativeembodiments.

In FIG. 3, the transform blocks 84 shall have the followingsignificance. Transformer 28 and inverse transformer 54 perform theirtransformations in units of these transform blocks 84. For instance,many codecs use some sort of DST or DCT for all transform blocks 84.Some codecs allow for skipping the transformation so that, for some ofthe transform blocks 84, the prediction residual signal is coded in thespatial domain directly. However, in accordance with embodimentsdescribed below, encoder 10 and decoder 20 are configured in such amanner that they support several transforms. For example, the transformssupported by encoder 10 and decoder 20 could comprise:

-   -   DCT-II, where DCT stands for Discrete Cosine Transform    -   DST-IV, where DST stands for Discrete Sine Transform    -   DCT-IV    -   DST-VII    -   Identity Transformation

Naturally, while transformer 28 would support all of the forwardtransform versions of these transforms, the decoder 20 or inversetransformer 54 would support the corresponding backward or inverseversions thereof:

-   -   Inverse DCT-II    -   Inverse DST-IV    -   Inverse DCT-IV    -   Inverse DST-VII    -   Identity Transformation

The subsequent description provides more details on which transformscould be supported by encoder 10 and decoder 20. In any case, it shouldbe noted that the set of supported transforms may comprise merely onetransform such as one spectral-to-spatial or spatial-to-spectraltransform.

As already outlined above, FIGS. 1 to 3 have been presented as anexample where the concept described further below may be implemented inorder to form specific examples for encoders 10 and decoders 20according to the present application. Insofar, the encoder 10 anddecoder 20 of FIGS. 1 and 2, respectively, represent possibleimplementations of the encoders and decoders described herein below.FIGS. 1 and 2 are, however, only examples. An encoder according toembodiments of the present application may, however, perform block-basedencoding of a picture 12 using the concept outlined in more detail belowand being different from the encoder of FIG. 1 such as, for instance, inthat same is no video encoder, but a still picture encoder, in that samedoes not support inter-prediction, or in that the sub-division intoblocks 80 is performed in a manner different than exemplified in FIG. 3,or even in that this encoder does not use transform prediction residualcoding with coding the prediction residual, for instance, in spatialdomain directly instead. Likewise, decoders according to embodiments ofthe present application may perform block-based decoding of picture 12′from data stream 14 using the concept further outlined below, but maydiffer, for instance, from the decoder 20 of FIG. 2 in that same is novideo decoder, but a still picture decoder, in that same does notsupport intra-prediction, or in that same sub-divides picture 12′ intoblocks in a manner different than described with respect to FIG. 3and/or in that same does not derive the prediction residual from thedata stream 14 in transform domain, but in spatial domain, for instance.

FIG. 4A shows an embodiment of an apparatus 10 for encoding a picture 12into a data stream 14. The picture 12 may be subdivided into blocks, orin case of e.g. HEVC-based codecs, subdivided into macroblocks, alsoreferred to as CTUs, 101 a, 101 b, . . . , 101 o.

The apparatus 10, in the following also referred to as encoder 10, isconfigured to further partition the macroblocks 101 a, 101 b, . . . ,101 o into smaller subblocks 102 a, 102 b, 102 c, 102 d, and finallyinto leaf blocks. The encoder 10 is configured to partition the picture12 into leaf blocks by using a recursive multi-tree partitioning. Inorder to further process said leaf blocks, the encoder 10 is configuredto block-based encode the picture 12 into the data stream 14, i.e. byencoding the leaf blocks.

Said block-based encoding of the picture 12 into the data stream 14 maycomprise a respective signaling in the data stream 14, which signalingmay comprise, inter alia, an indication regarding the partitioning intothe data stream 14, for example an indication for indicating one or morepartitioning parameters, such as a split mode and a split ratio beingused for a partitioning of a current block.

With respect to the multi-tree partitioning, the blocks may bepartitioned along a splitting tree, wherein each node of the splittingtree may belong to a respective block in the picture. If a block issplit, the split tree may succeed to a subsequent tree node at asubsequent split level, also referred to as a partitioning level. Theabove mentioned signaling in the data stream 14 may be done at one ormore partitioning levels. According to some examples, partitioningparameters may be signaled in the data stream 14 at one or morepartitioning levels, and in some other examples partitioning parametersmay be signaled in the data stream 14 at each partitioning level.According to a further example, partitioning parameters may be signaledin the data stream 14 at a partitioning level in the split tree, whereinthe tree nodes at said partitioning level may belong to one or moreblocks which are located at a picture boundary of the picture 12 to beencoded. The above described signaling in the data stream 14 may be usedin connection with each of the inventive concepts and correspondingembodiments as described herein.

Returning to FIG. 4A, it can be seen that the macroblocks 101 a, 101 b,. . . , 101 o are quadratic structures of a predetermined size spanninga fixed grid over the picture 12. Since this fixed grid may exceed thesize of the picture 12, it may sometimes happen that some of themacroblocks 101 a, 101 b, . . . , 101 o of said grid may exceed theboundaries of the picture 12. In FIG. 4A, such a scenario is exemplarilyshown by means of macroblocks 101 e, 101 m and 101 o.

For example with respect to macroblock 101 e, an upper left portion anda lower left portion of macroblock 101 e may be located inside thepicture 12, while an upper right portion and a lower right portion ofmacroblock 101 e may be located outside the picture 12. Accordingly, themacroblock 101 e extends beyond a boundary of the picture 12 such thatsaid boundary of the picture 12 crosses the macroblock 101 e at acertain position, namely at such a position according to which an upperleft portion and a lower left portion of the macroblock 101 e arelocated inside the picture 12, while an upper right portion and a lowerright portion of the macroblock 101 e are located outside the picture12.

For example with respect to macroblock 101 o, only an upper left portionof the macroblock 101 o is located inside the picture 12, while topright, bottom right and bottom left portions of macroblock 101 o arelocated outside the picture 12. Furthermore, regarding macroblock 101 m,a top left portion and a top right portion of macroblock 101 m arelocated inside the picture 12, while a bottom left and a bottom rightportion of macroblock 101 m are located outside the picture 12.Accordingly, the respective boundary of the picture 12 crosses therespective macroblocks 101 e, 101 m, 101 o at different positions. Moredetails about the positions of the macroblocks, and in particular withrespect to positions of the macroblocks 101 e, 101 m, 101 o relative tothe boundaries of the picture 12 will be explained somewhat later in thetext with reference to FIG. 6.

As mentioned above, the macroblocks 101 a, 101 b, . . . , 101 o may bepartitioned into smaller subblocks and finally into leaf blocks. Anon-limiting example is shown with respect to subblocks 102 a, 102 b,102 c, 102 d. Subblocks may themselves be subjected to partitioninguntil a leaf block is reached. The partitioning may also be referred toas splitting, wherein such splitting of macroblocks and subblocks may beexecuted by using different split modes. Examples may be quad splits,horizontal binary splits, vertical bi-splits, horizontal ternary splitsand vertical ternary splits. Split signaling might be performed using abinary decision tree, wherein said split signaling may be indicated inthe data stream 14 in a way as previously described above.

For a brief explanation of such a decision tree, it shall be referred toFIGS. 5A to 5D, which Figures compare a split signaling at a boundary ofa picture with a split signaling inside a picture. FIGS. 5A and 5Cdepict a fully unrestricted decision tree that may be used for splitsignaling inside a picture, while FIGS. 5B and 5D depict an exemplaryrestricted decision tree according to the present concept that may beused for split signaling at a picture boundary.

The fully unrestricted decision tree in FIG. 5A may be used in thepresent concept as an available set of split modes that may, forinstance, be based on a Quad Tree plus Binary Tree structure asdescribed in [2]. The QTBT scheme comprises an available set of splitmodes for splitting a block into a smaller block which available setcontains the four depicted split modes, namely a quad split, no split, avertical split or a horizontal split. Accordingly, upon eachpartitioning of a block into a smaller block, the known encoder mayalways have to step through the fully unrestricted decision tree, i.e.through the full set of available split modes. As can be seen, forreaching the horizontal or vertical split in the decision tree, threebins are spent. The encoder further has to signal the respectivelyselected split mode in the data stream. At a picture boundary, however,the known technology may perform an implicit quad split and refrain fromsignaling any related indication in the data stream. This results in aquite inflexible handling of blocks at a picture boundary.

The fully unrestricted decision tree in FIG. 5C may be used in thepresent concept as an available set of split modes that may, forinstance, be based on a Quad Tree plus Binary Tree plus aMulti-Type-Tree structure as described in [2] and [3]. The QTBT+MTTscheme comprises an available set of split modes for splitting a blockinto a smaller block which available set contains the six depicted splitmodes, namely a quad split, no split, a vertical binary split, avertical ternary split, a horizontal binary split and a horizontalternary split. Accordingly, upon each partitioning of a block into asmaller block, the known encoder may always have to step through thefully unrestricted decision tree, i.e. through the full set of availablesplit modes. In this example, for reaching the horizontal or verticalsplit in the decision tree, four bins are spent. The encoder further hasto signal the respectively selected split mode in the data stream. At apicture boundary, however, the known technology may perform an implicitquad split and refrain from signaling any related indication in the datastream. This results in an even more inflexible handling of blocks at apicture boundary compared to FIG. 5A.

Thus, according to the invention, the encoder 10 is configured to reducean available set of split modes to provide a reduced set of split modes,at least for those predetermined blocks, that are located at a pictureboundary, i.e. at least partially inside the picture 12, as exemplarilydescribed before with reference to the predetermined blocks 101 e, 101 mand 101 o shown in FIG. 4A.

As exemplarily shown in FIGS. 5B and 5D, the aforementioned reduced setof split modes may, for instance, comprise exactly two split modes.Alternatively, a reduced set of split modes may comprise more than twosplit modes, or exactly one split mode.

For example, a reduced set of split modes may comprise a quad split anda vertical split. The vertical split may, for instance be a verticalbinary split or a vertical ternary split.

Additionally or alternatively, the reduced set of split modes may, forinstance, comprise a quad split and a horizontal split. The horizontalsplit may, for instance be a horizontal binary split or a horizontalternary split.

Additionally or alternatively, the reduced set of split modes may, forinstance, comprise a vertical split and a horizontal split. Thehorizontal split may, for instance be a horizontal binary split or ahorizontal ternary split, and the vertical split may, for instance, be avertical binary split or a vertical ternary split.

Additionally or alternatively, the reduced set of split modes may, forinstance, comprise a quad-split only.

Additionally or alternatively, the reduced set of split modes may, forinstance, comprise either a horizontal split or a vertical split, forexample in case a quad-split may not be available.

Additionally or alternatively, the reduced split modes may, forinstance, comprise exactly one predetermined split mode which can eitherbe a horizontal or a vertical split. Which split to choose may depend onthe position of the predetermined block relative to the respectivepicture boundary, e.g. the horizontal split may be selected in case ofthe bottom picture boundary, while the vertical split may be selected incase of the right picture boundary. The horizontal split may be ahorizontal bi-split or a horizontal ternary split. The vertical splitmay be a vertical bi-split or a horizontal ternary split. As can be seenin the decision tree, e.g. in FIG. 5D, only one of the horizontal orvertical split may be available in the reduced set.

In each of the above cases, one or more of the binary splits may, forexample, comprise a split-ratio of 1/2.

Summarizing in more general terms, a reduced split set according to theherein described innovative principle may comprise the following splits:

-   -   (a) If such a split exists, an unrestricted split which will,        when applied, not obstruct further sub-partitioning of a given        block.    -   (b) If such a split exists, a split which will create an edge        parallel to the picture boundary, and        -   i. if such a split exists and is allowed for a given block,            a split placed in the middle of the split block, or        -   ii. if such a split exists and is allowed for a given block,            a split which will create a partitioning edge collocated            with the picture boundary.

In a particular non-limiting embodiment of this invention based on QTBT,but also valid for [3] and [4] as well as other partitioning schemesproviding the above mentioned different split possibilities: quad-split,vertical ternary split, vertical binary split, horizontal ternary split,and horizontal binary split; reduced split sets according to theinnovative principle described herein may be:

-   -   Quad-split and vertical bi-split, e.g. with split ratio 1/2-i or        vertical ternary split-ii    -   Quad-split and horizontal bi-split, e.g. with split ratio 1/2-i,        or horizontal ternary split-ii    -   Quad-split or either horizontal or vertical binary or ternary        split, e.g. with split ratio 1/2.

Accordingly, a reduced set of split modes according to the innovativeprinciple may comprise pre-selected ones of split modes for splitting ablock at a picture boundary. The encoder 10 may select one of the splitmodes contained in the reduced set of split modes for partitioning theblock at the picture boundary, and the encoder 10 may signal theselected split mode in the data stream 14. Thus, the present concept mayextend the signaling for blocks at picture boundaries when compared toimplicit quad splits in the known technology. However, the encoder 10may only have to signal the uncertainty in the data stream 14 whichkeeps the signaling effort at a considerable level. However, the presentconcept provides for a much higher flexibility in partitioning blocks ata picture boundary compared to the known technology. This gain inflexibility may compensate and/or outweigh the slightly higher signalingeffort.

Even though FIG. 5B may exemplarily depict a reduced set of split modescomprising two different split modes, namely a quad split and a verticalbi-split, the reduced set may also comprise a quad split and ahorizontal bi-split, or a vertical bi-split and a horizontal bi-split,or any other combination as discussed above. According to an example,the reduced set of split modes may comprise at least one split mode.According to a further example, the reduced set of split modes maycomprise at least two different split modes. According to yet a furtherexample, the reduced set of split modes may comprise exactly twodifferent split modes only, even though an available set of split modesmay contain more than these two different split modes.

The selection of the split modes that shall be contained in the reducedset of split modes may depend on the position of the respectivepredetermined block 101 e, 101 m, 101 o relative to the boundary of thepicture 12, i.e. depending on a position at which the boundary of thepicture 12 crosses the predetermined block 101 e, 101 m, 101 o.

In other words, the encoder 10 is configured to, in partitioning thepicture 12 into the leaf blocks, for a predetermined block 101 e, 101 m,101 o that corresponds to a predetermined tree level of the multi-treepartitioning and which extends beyond a boundary of the picture 12,reduce an available set of split modes for splitting the predeterminedblock 101 e, 101 m, 101 o depending on a position at which the boundaryof the picture 12 crosses the predetermined block 101 e, 101 m, 101 o toobtain a reduced set of one or more split modes.

According to the invention, if a cardinality of the reduced set is one,i.e. if the reduced set of split modes may comprise only one split mode,the encoder 10 is configured to apply this split mode of the reduced setfor splitting the predetermined block 101 e, 101 m, 101 o.

However, if a cardinality of the reduced set is greater than one, i.e.if the reduced set of split modes comprises two or more split modes, theencoder 10 is configured to select one of these split modes of thereduced set and to apply the selected one of the split modes forsplitting the predetermined block 101 e, 101 m, 101 o. Furthermore, theencoder 10 is configured to signal the respective selection, i.e. theselected split mode, in the data stream 14 in a way as explained above.For example, the encoder 10 may signal the selection by setting a 1-bitflag. However, as described above, only the uncertainty between thesplit modes that are contained in the already reduced set of split modesmay have to be signaled in the data stream 14.

As mentioned above, the cardinality of the reduced set may, for example,be exactly one or exactly two.

FIG. 4B shows the corresponding apparatus 20 for decoding the picture 12in the data stream 14, said apparatus being also referred to in thefollowing as a decoder 20.

The decoder 20 is configured to partition the picture 12 into leafblocks using recursive multi-tree partitioning, and to block-baseddecode the picture 12 from the data stream 14 using the partitioning ofthe picture 12 into leaf blocks.

The decoder 20 is further configured to, in partitioning the picture 12into the leaf blocks, for a predetermined block that corresponds to apredetermined tree level of the multi-tree partitioning and whichextends beyond a boundary of the picture 12, reduce an available set ofsplit modes for splitting the predetermined block depending on aposition at which the boundary of the picture 12 crosses thepredetermined block to obtain a reduced set of one or more split modes.

Again, if a cardinality of the reduced set is one, i.e. if the reducedset of split modes may comprise only one split mode, the decoder 20 isconfigured to apply the split mode of the reduced set for splitting thepredetermined block.

If a cardinality of the reduced set is greater than one, i.e. if thereduced set of split modes comprises two or more split modes, thedecoder 20 is configured to select one of the split modes of the reducedset and to apply the selected one of the split modes for splitting thepredetermined block according to a signalization in the data stream 14.That is, the decoder 20 may derive a signal from the data stream 14indicating towards the decoder 20 which split mode to use for thepartitioning. This signal may be the above mentioned 1-bit flag that mayhave been previously packed into the data stream by the encoder 10.

Accordingly, the concept as described herein provides a novel mechanismof boundary handling. It may be applicable to all recursive partitioningmethods that generally allow more than one split at a specific level.Again, the present concept may extend the signaling for blocks atpicture boundaries when compared to implicit splits of the knowntechnology. However, the present concept provides for a much higherflexibility in partitioning blocks at a picture boundary compared to theknown technology. This gain in flexibility may compensate and/oroutweigh the slightly higher signaling effort of the reduced set ofsplit modes according to the present concept.

A non-limiting example of the present concept is depicted in FIG. 6which shows a partitioned picture 12 similar to FIG. 4A. However, FIG. 6shows some more details, namely a picture 12 having a size w_(f)×h_(f),which picture 12 is subdivided into blocks, for example macroblocks 101a, 101 b, . . . , 101 o, smaller subblocks and finally leaf blocks.Three particular macroblocks are highlighted and numbered with encirclednumbers 1, 2 and 3. These exemplary highlighted macroblocks maycorrespond to previously described macroblocks 101 e, 101 m, 101 o asshown in FIG. 4. Furthermore, leaf blocks will be discussed in moredetail somewhat later in the text with reference to FIGS. 8A and 8B.

FIG. 6 shows a partitioning of the picture 12 into rigid macroblockstructure of a fixed size. In this example it may be assumed that thevideo signal resolution may not be spatially restricted to multiples ofthe size of a single macroblock 101 a, 101 b, . . . , 101 o. If thoseprerequisites are given, it may be inferred that the last macroblock ofa macroblock line and/or all of the macroblocks in the last macroblockline might exceed at least one boundary of the picture 12. In such acase, the low-level partitioning of this macroblock into coding units,prediction units, and transform units or any other type of sub-CTU-levelpartitions should ensure that the coded partitions are located insidethe boundaries of the picture 12. The concept as described herein mayallow for a definition of reduced split sets for partitioning blocks,for example macroblock 101 m being defined by position x_(b), y_(b),having a width W_(b), and a height h_(b), which blocks may exceed theboundaries of the picture 12 of size w_(f)×h_(f) in at least one of thefollowing six cases:

-   -   C1: Corresponding to block 101 e, i.e. the top right corner of        the block 101 e is not contained in the picture 12, the bottom        left corner is contained in the picture 12: x_(b)+w_(b)>w_(f);        y_(b)+h_(b)≤h_(f); x_(b)≥0; y_(b)≥0    -   C2: Corresponding to block 101 m, i.e. the top right corner of        the block 101 m is contained in the picture 12, the bottom left        corner is not contained in the picture 12: x_(b)+w_(b)≤w_(f);        y_(b)+h_(b)>h_(f); x_(b)≥0; y_(b)≥0    -   C3: Corresponding to block 101 o, i.e. both the top right and        the bottom left corners are not contained in the picture 12:        x_(b)+w_(b)>w_(f); y_(b)+h_(b)>h_(f); x_(b)≥0; y_(b)≥0    -   C4: The top left corner of a block is not contained in the        picture, the bottom left corner is not contained in the picture        12, the top right corner is contained in the picture 12:        x_(b)<0; y_(b)≥0; x_(b)+w_(b)>0    -   C5: The top left corner of a block is not contained in the        picture 12, the top right corner is not contained in the picture        12, the bottom left corner is contained in the picture 12:        Y_(b)>0; y_(b)<0; y_(b)+h_(b)>0    -   C6: Only one of the following block corners is contained in the        picture 12: bottom right, top right or bottom left

If none of the cases C1 to C6 is true for a specific predetermined blockand picture 12, the predetermined block may lie outside of theboundaries of the picture 12 and may be of no further interest to theconcept as described herein. More than one case can be true at once,specifically, case pairs C1 and C4, C2 and C5 or C3 and C6 can be truesimultaneously. As those case pairs will all be handled equivalently,the specific constellations do not need to be handled separately.

For handling at least one of these cases C1 to C6, the present conceptprovides a reduced set of split modes that may comprise one or moresplit modes selected out of an actually available set of split modes.

This actually available set of split modes may be chosen by the encoder10 depending on the used block-based coding scheme. According to anexample, the encoder 10 may be configured to determine the available setof split modes for the predetermined block

-   -   independent from a location of the predetermined block relative        to the picture boundary, and/or    -   dependent on a sequence of split modes of preceding tree levels        from which the predetermined block emerges.

The same holds true for the decoder 20. According to an example, thedecoder 20 may be configured to determine the available set of splitmodes for the predetermined block

-   -   independent from a location of the predetermined block relative        to the picture boundary, and/or    -   dependent on a sequence of split modes of preceding tree levels        from which the predetermined block emerges.

Again, for handling at least one of the above mentioned cases C1 to C6,the present concept provides a reduced set of split modes that maycomprise one or more split modes selected out of the actually availableset of split modes.

For example, a reduced split set may consist of the following splits:

-   -   (a) Provided that such a split exists: an unrestricted split        that will, when applied, not impose restrictions on further        splitting of a given block.    -   (b) For cases C1 and C2, provided that such a split exists: a        split that will create an edge parallel to the boundary of the        picture 12.        -   i. Provided that such a split exists and that it is allowed            for a given block: a split placed in the middle of the split            block.        -   ii. Provided that such a split exists and that it is allowed            for a given block: a split which will create a partitioning            edge collocated with the boundary of the picture 12.

In other words, the encoder 10 may be configured to, for reducing theavailable set of split modes to the reduced set of one or more splitmodes, select from the available set of split modes at least one of:

-   -   a first split mode comprising a split above) which does not        impose any restriction onto one or more subsequent splits for        subsequent tree levels succeeding the predetermined tree level,        and    -   a second split mode comprising a split that splits the        predetermined block into two or three subblocks along at least        one split line that is parallel to the boundary's position in        the predetermined block and which split line is at least one of        centered inside the predetermined block-i above) or collocated        to the boundary of the picture 12-ii above).

The same holds true for the decoder 20, respectively. That is, thedecoder 20 may be configured to, for reducing the available set of splitmodes to the reduced set of one or more split modes, select from theavailable set of split modes at least one of:

-   -   a first split mode comprising a split above) which does not        impose any restriction onto one or more subsequent splits for        subsequent tree levels succeeding the predetermined tree level,        and    -   a second split mode comprising a split that splits the        predetermined block into two or three subblocks along at least        one split line that is parallel to the boundary's position in        the predetermined block and which split line is at least one of        centered inside the predetermined block-i above) or collocated        to the boundary of the picture 12-ii above).

If a split-ii is not available in the used partitioning method for aspecified block, split-i may be included in the reduced split set.Split-i might be generally of advantage over split-ii. As mentionedabove, at least one split may be included in the reduced split set.

According to an example, split may be a quad split and split-i may be atleast one of a horizontal bi-split or a vertical bi-split. According toa further example, one of the additionally introduced splits asdescribed in [3] and [4] could be used for-ii if the boundary of thepicture 12 lies in 1/4 or 3/4 of the partitioned block. In a particularimplementation a vertical or horizontal split having a split ratio ofI/2 could be of advantage over additional splits introduced in [3]and/or [4].

The above mentioned splits are schematically shown in FIGS. 7A to 7E.FIG. 7A shows a quad split wherein a predetermined block 701 is splittedinto four equal partitions 71 a, 71 b, 71 c, 71 d. FIG. 7B shows avertical bi-split with splitting ratio 1/2 wherein a predetermined block702 is splitted into two equal vertical partitions 72 a, 72 b. FIG. 7Cshows a horizontal bi-split with splitting ratio 1/2 wherein apredetermined block 703 is splitted into two equal horizontal partitions73 a, 73 b. FIG. 7D shows a vertical ternary split as described in [3]wherein a predetermined block 704 is splitted into three verticalpartitions 74 a, 74 b, 74 c. FIG. 7E shows a horizontal ternary split asdescribed in [3] wherein a predetermined block 705 is splitted intothree horizontal partitions 75 a, 75 b, 75 c.

According to an example, the vertical bi-split as shown in FIG. 7B maybe applied if the respective boundary of the picture 12 crosses therespective predetermined block 702 such that the boundary of the picture12 that is located inside the respective block 702 is also vertical,i.e. parallel to the vertical split line. Such a case may, for instance,be true for block 101 e as shown in FIG. 6.

The horizontal bi-split as shown in FIG. 7C may be applied if therespective boundary of the picture 12 crosses the respectivepredetermined block 703 such that the boundary of the picture 12 that islocated inside the respective block 703 is also horizontal, i.e.parallel to the horizontal split line. Such a case may, for instance, betrue for block 101 m as shown in FIG. 6.

According to yet a further example which may make use of a GeneralizedBinary Splitting, split may be a perpendicular split having a splitratio of 1/2 or a quad split, if the Generalized GBS tree was used as asubtree to a quad-tree structure. Vertical and horizontal splits,possibly translated into perpendicular/parallel semantics may be used assplit-i or-ii. Again, splits with the modifier 1/2 could be of advantageeven if other splits would provide a split collocated with the boundaryof the picture 12.

In yet a further example, which may be based on QTBT, but which may alsobe valid for the schemes as described in [3] and [4] as well as for anyother partitioning schemes providing the following three splitpossibilities: quad split, vertical split with a split ratio of 1/2 anda horizontal split with a split ratio of 1/2 three exemplary reducedsplit sets may be:

-   -   RSS1: Quad-split and vertical split with split ratio 1/2-i    -   RSS2: Quad-split and horizontal split with split ratio 1/2-i    -   RSS3: Quad-split or either vertical or horizontal split with        split ratio 1/2-i.

These exemplary reduced sets of split modes, i.e. RSS1, RSS2 and RSS3,may be one non-limiting example for a subset of an available set ofsplit modes. That is, a reduced set of split modes may comprise one ormore available splits being available in an available set of splitmodes. The number of splits contained in an available set of split modesmay vary depending on the coding scheme, while the number of splitscontained in a reduced set of split modes may vary depending on theposition of the respective predetermined block relative to the boundaryof the picture 12.

As mentioned above, the encoder 10 may determine an available set ofsplit modes. That is, the encoder 10 may selectively choose which splitmodes shall be contained in the available set of split modes.

For example, the above mentioned block-based coding schemes may comprisea primitive set of split modes including a quad split, at least onehorizontal bi-split and at least one vertical bi-split.

However, some of the splits, and in particular some of the bi-splits,may be restrictive. For example, if a bi-split was applied to apredetermined block, then a quad split may not be allowed as asubsequent split. In other words, a quad split may only be allowed at acertain tree level if preceding splits of previous tree levels did notcontain one of the restrictive splits, and in particular one of therestrictive bi-splits.

Accordingly, if one of the restrictive splits, and in particular one ofthe restrictive bi-splits, has been applied to a predetermined block ata predetermined tree level, then subsequent splits may only contain theabove mentioned primitive set but without the quad split.

Thus, according to an embodiment, the encoder 10 may be configured todetermine the available set of split modes

-   -   to be equal to a primitive set of split modes including a quad        split, at least one horizontal bi-split and at least one        vertical bi-split, if all split modes of preceding tree levels        from which the predetermined block emerges are outside a        restrictive set of bi-splits, and    -   to be equal to the primitive set of split modes less the quad        split, if a split mode of a preceding tree level from which the        predetermined block emerges is one of the restrictive set of        bi-splits.

The same holds true for the decoder 20. Accordingly, the decoder 20 maybe configured to determine the available set of split modes

-   -   to be equal to a primitive set of split modes including a quad        split, at least one horizontal bi-split and at least one        vertical bi-split, if all split modes of preceding tree levels        from which the predetermined block emerges are outside a        restrictive set of bi-splits, and    -   to be equal to the primitive set of split modes less the quad        split, if a split mode of a preceding tree level from which the        predetermined block emerges is one of the restrictive set of        bi-splits.

Additionally or alternatively, the above mentioned primitive set ofsplit modes may include at least one horizontal ternary split and atleast one vertical ternary split.

As mentioned above, the encoder 10 may reduce available sets of splitmodes so as to provide a reduced set of split modes. Therefore, theencoder 10 may selectively choose, for each predetermined block, areduced subset of split modes out of the above mentioned available setof split modes. Only these selectively chosen split modes may then becontained in the reduced set of split modes. Thus, the reduced set ofsplit modes may comprise at least one of the above mentioned quad splitaccording to split, and one of the splits according to split-i orsplit-ii.

Thus, according to an embodiment, the encoder 10 may be configured to,for reducing the available set of split modes to the reduced set of oneor more split modes, select from the available set of split modes atleast one of:

-   -   the quad split, and    -   a bi-split that splits the predetermined block 702, 703 into two        subblocks 72 a, 72 b, 73 a, 73 b along a split line that is        parallel to the picture boundary's position in the predetermined        block 702, 703 and which split line is at least one of centered        inside the predetermined block according to split-i and        collocated to the boundary according to split-ii.

Again, the same holds true for the decoder 20. Accordingly, the decoder20 may be configured to, for reducing the available set of split modesto the reduced set of one or more split modes, select from the availableset of split modes at least one of:

-   -   the quad split, and    -   a bi-split that splits the predetermined block 702, 703 into two        subblocks 72 a, 72 b, 73 a, 73 b along a split line that is        parallel to the picture boundary's position in the predetermined        block 702, 703 and which split line is at least one of centered        inside the predetermined block according to split-i and        collocated to the boundary according to split-ii.

Again, split-i may comprise a bi-split, while split-ii may comprise abi-split and/or a ternary split.

Thus, according to an embodiment, the encoder 10 may be configured to,for reducing the available set of split modes to the reduced set of oneor more split modes, select from the available set of split modes atleast one of:

-   -   the quad split, and    -   a ternary split that splits the predetermined block 704, 705        into three subblocks 74 a, 74 b, 74 c, 75 a, 75 b, 75 c along a        split line that is parallel to the picture boundary's position        in the predetermined block 704, 705 and which split line is        collocated to the boundary according to split-ii.

Again, the same holds true for the decoder 20. Accordingly, the decoder20 may be configured to, for reducing the available set of split modesto the reduced set of one or more split modes, select from the availableset of split modes at least one of:

-   -   the quad split, and    -   a ternary split that splits the predetermined block 704, 705        into three subblocks 74 a, 74 b, 74 c, 75 a, 75 b, 75 c along a        split line that is parallel to the picture boundary's position        in the predetermined block 704, 705 and which split line is        collocated to the boundary according to split-ii.

FIGS. 8A and 8B show an example of how to apply the herein describedconcept to a block based coding scheme together with picture boundaryhandling.

FIG. 8A shows the partitioning of a predetermined block that may, inthis example, correspond to macroblock 101 o as shown in FIG. 6. Thehatched parts of the block 101 o represent the position of the lowerright corner of the picture 12. This example shows a partitioning of themacroblock 101 o into smaller subblocks by applying quad splits only.

The numbers indicated in the respective subblocks may indicate thenumber of quad splits that have to be applied so as to arrive at theleaf-block partitioning of the macroblock 101 o as it is depicted inFIG. 8A. A first quad split divides the macroblock 101 o into four equalsubblocks 801 a, 801 b, 801 c, 801 d. As can be seen, only the left twosubblocks 801 a, 801 d may still comprise the boundary of the picture12. The right two subblocks 801 b, 801 c are located outside of thepicture 12 and do not comprise the boundary of the picture 12 anymore.They can be discarded.

A second quad split divides the left two subblocks 801 a, 801 d intofour smaller subblocks again. As can be seen, the left two subblocks 802a, 802 b are located inside the picture 12 and do not contain thepicture boundary anymore. Accordingly, these two subblocks 802 a, 802 bcan be further partitioned using unrestricted split sets according tothe used partitioning method. Further partitioning of those blocks maynot be described in further detail herein with respect to the presentconcept.

However, the remaining subblocks have to be partitioned for a third timesince all of them may still contain the picture boundary. Said thirdpartitioning leads to leaf blocks which are indicated by numbers ‘3’.

FIG. 8B shows an alternative partitioning which applies the hereindescribed concept of reduced split sets. The numbers in each subblockindicates the number of quad splits and non-quad splits that have to beapplied so as to arrive at the leaf-block partitioning of macroblock 101o as shown in FIG. 8B. The first number may indicate the number of quadsplits, and the second number may indicate the number of non-quadsplits.

For example, for the upper left leaf block 802 a one quad split and onevertical bi-split was applied. For the adjacent right leaf block 802 bone quad split and two vertical bi-splits were applied. For the adjacentlower leaf block 802 c two quad splits and one horizontal bi-split wasapplied. For the leaf block 802 d in the lower right corner of thepicture 12, three quad splits and none non-quad split were applied.

In FIG. 8B the solid lines indicate edges created by a quad split, thedashed lines indicate edges created by a vertical bi-split, and thedashed-dotted-line indicates an edge created by a horizontal bi-split.

In more detail, as a first split at a first tree level a quad split maybe applied which results in four equal subblocks 801 a, 801 b, 801 c,801 d. As can be seen, only the left two subblocks 801 a, 801 d maystill comprise the boundary of the picture 12. The right two subblocks801 b, 801 c are located outside of the picture 12 and do not comprisethe boundary of the picture 12 anymore. They can be discarded.

The upper left subblock 801 a may now be further partitioned at a secondtree level. For the partitioning of subblock 801 a a reduced split setmay be provided. For example, a reduced split set comprising at leastone of a quad split and a vertical bi-split, or comprising a verticalbi-split only, may be provided. In the example shown in FIG. 8B, thesubblock 801 a may be subjected to a vertical bi-split with a modifier1/2 at the second tree level, wherein the split line 811 is centeredinside the block 801 a. Accordingly, the upper left subblock 802 a maybe created in result.

With the present concept, only one quad split and one vertical bi-splithave to be applied to arrive at subblock 802 a which is a leaf block. InFIG. 8A instead, two blocks 802 a, 802 b are used to cover the same areaof the picture 12 as the only one block 802 a of FIG. 8B.

Furthermore, the right adjacent block 802 b in FIG. 8B may now befurther partitioned at a third tree level. For the partitioning ofsubblock 802 b a reduced split set may be provided again. For example, areduced split set comprising at least a vertical bi-split may beprovided, since the previous split was already a vertical bi-split suchthat a subsequent quad split may not be allowed. In the example shown inFIG. 8B, the subblock 802 b may be subjected to a vertical bi-split witha modifier 1/2 at the third tree level, wherein the split line 812 iscentered inside the block 802 b and collocated with the pictureboundary. Accordingly, the left subblock 803 a may be created in resultat the third tree level, wherein said left subblock 803 a is a leafblock, but it might also by further partitioned with the usedpartitioning method.

With the present concept, only one quad split and two vertical bi-splitshave to be applied to arrive at subblock 803 a which is a leaf block. InFIG. 8A instead, four blocks are used to cover the same area of thepicture 12 as the only one block 803 a of FIG. 8B. However, if theencoder 10 decides so, partitioning 8A might just as well be applied ifa quad-split is always selected from the reduced split sets.

Furthermore, the lower adjacent block 802 d in FIG. 8B may derive fromtwo consecutive quad splits at a first and a second tree level,respectively, and block 802 d may now further be partitioned at a thirdtree level. For the partitioning of subblock 802 d a reduced split setmay again be provided. For example, a reduced split set comprising atleast a quad split and a horizontal bi-split, or a reduced split setcomprising a horizontal bi-split only, may be provided. In the exampleshown in FIG. 8B, the subblock 802 d may be subjected to a horizontalbi-split with a modifier 1/2 at the third tree level, wherein the splitline 813 is centered inside the block 802 d and collocated with thepicture boundary. Accordingly, subblock 803 b may be created in resultat the third tree level, wherein said subblock 803 b is a leaf block.

With the present concept, only two quad splits and one horizontalbi-split have to be applied to arrive at subblock 803 b which is a leafblock. In FIG. 8A instead, two blocks are used to cover the same area ofthe picture 12 as the only one block 803 b of FIG. 8B.

Block 803 c may also be a leaf block that may be derived from threeconsecutive quad splits. This example of leaf block 803 c may beidentical to the quad split partitioning of the same block shown in FIG.8A. The syntax may only allow to provide quad splits in a reduced set ofsplit modes because the corner of the picture 12 is contained in theblocks leading to leaf block 803 c.

According to an example, the encoder 10 may be configured to, forreducing the available set of split modes to the reduced set of one ormore split modes, select from the available set of split modes the quadsplit only if the predetermined block extends beyond a corner of thepicture 12.

The same holds true for the decoder 20. Accordingly, the decoder 20 maybe configured to, for reducing the available set of split modes to thereduced set of one or more split modes, select from the available set ofsplit modes the quad split only if the predetermined block extendsbeyond a corner of the picture 12.

As shown in FIG. 8B, the resulting leaf blocks 802 a, 803 a, 803 b, 803c represent a minimal partitioning used to handle the boundaries of thepicture 12. Additionally or alternatively, the resulting leaf blocks 802a, 803 a, 803 b, 803 c might be further split within the givenrestriction of the used partitioning method.

Generally, the reduced split sets might be subjected to furtherlimitations, provided that the final contains at least one valid split.Those limitations might be, but are not limited to:

-   -   Split availability for a specified block size    -   Split availability for a specified depth.

The latter bullet point “Split availability for a specified depth” shallbe briefly explained by means of the following example, and again withreference to FIGS. 5A and 5B:

To ensure optimal efficiency, although some signaling is used instead ofpurely implicit split derivation, only limited information needs to besignaled. Most notably no information about splitting itself needs to betransferred. In the above discussed picture boundary cases C1 to C3 asplit will be performed. If the reduced split set contains more than oneelement, only the remaining uncertainty needs to be signaled. In aparticular embodiment based on QTBT described above, one binary signalwould need to be transferred for cases C1 and C2, and no signaling wouldbe required in case C3 because C1 may comprise a reduced split setcomprising a quad split and a vertical bi-split, C2 may comprise areduced split set comprising a quad split and a horizontal bi-split, andC3 may comprise a reduced split set comprising a quad split only.

Further syntax restrictions may apply in the present concept. Forexample, if a split causes additional restrictions to arise, those mightbe fully or partially discarded if the split block overlaps the pictureboundary. According to an example, the number of subsequent binarysplits may be restricted.

If a binary split is applied over a picture boundary as a part of areduced set, it might not be counted in the enforcement of thisrestriction.

Thus, according to an example, if a bi-split from the reduced set isapplied and the number of consecutive bi-splits is restricted to apredetermined maximum number of consecutive bi-splits, the encoder 10may be configured to not count a consecutive bi-split if saidconsecutive bi-split is applied over a picture boundary.

The same holds true for the decoder 20. Accordingly, if a bi-split fromthe reduced set is applied and the number of consecutive bi-splits isrestricted to a predetermined maximum number of consecutive bi-splits,the decoder 20 may be configured to not count a consecutive bi-split ifsaid consecutive bi-split is applied over a picture boundary.

Other restrictions, e.g. forbidding a quad split to follow any splitother than a quad-split, might be further enforced, even over pictureboundaries.

FIG. 9 shows a block diagram of a method of encoding a picture 12according to the present concept.

In block 901 the picture is partitioned into leaf blocks using recursivemulti-tree partitioning.

In block 902 the picture is block-based encoded into a data stream usingthe partitioning of the picture into leaf blocks.

In block 903, for a predetermined block, an available set of split modesfor splitting the predetermined block is reduced, depending on aposition at which the boundary of the picture crosses the predeterminedblock to obtain a reduced set of one or more split modes.

If a cardinality of the reduced set is one, the split mode of thereduced set is applied for splitting the predetermined block, asdepicted by block 904.

If the cardinality of the reduced set is greater than one, one of thesplit modes of the reduced set is selected and the selected one of thesplit modes is applied for splitting the predetermined block, asdepicted by block 905.

In block 906, the selection, i.e. the selected one of the split modesfrom the reduced set of split modes, is signaled in the data stream.

FIG. 10 shows a block diagram of a method of decoding a picture 12according to the present concept.

In block 1001 the picture is partitioned into leaf blocks usingrecursive multi-tree partitioning.

In block 1002 the picture is block-based decoded from the data streamusing the partitioning of the picture into leaf blocks.

In block 1003, for a predetermined block, an available set of splitmodes for splitting the predetermined block is reduced, depending on aposition at which the boundary of the picture crosses the predeterminedblock to obtain a reduced set of one or more split modes.

If a cardinality of the reduced set is one, the split mode of thereduced set is applied for splitting the predetermined block, asdepicted by block 1004.

If the cardinality of the reduced set is greater than one, one of thesplit modes of the reduced set is selected and the selected one of thesplit modes is applied for splitting the predetermined block, asdepicted by block 1005.

As shown in block 1006, the selection of the one of the split modes fromthe reduced set of split modes is based on a signalization in the datastream.

The method steps, represented by the blocks in the block diagram of FIG.9 and FIG. 10, respectively, may also be executed in a different orderthan depicted.

Although some aspects have been described in the context of anapparatus, it is clear that these aspects also represent a descriptionof the corresponding method, where a block or device corresponds to amethod step or a feature of a method step. Analogously, aspectsdescribed in the context of a method step also represent a descriptionof a corresponding block or item or feature of a correspondingapparatus. Some or all of the method steps may be executed by a hardwareapparatus, like for example, a microprocessor, a programmable computeror an electronic circuit. In some embodiments, one or more of the mostimportant method steps may be executed by such an apparatus.

Depending on certain implementation requirements, embodiments of theinvention can be implemented in hardware or in software or at leastpartially in hardware or at least partially in software. Theimplementation can be performed using a digital storage medium, forexample a floppy disk, a DVD, a Blu-Ray, a CD, a ROM, a PROM, an EPROM,an EEPROM or a FLASH memory, having electronically readable controlsignals stored thereon, which cooperate with a programmable computersystem such that the respective method is performed. Therefore, thedigital storage medium may be computer readable.

Some embodiments according to the invention comprise a data carrierhaving electronically readable control signals, which are capable ofcooperating with a programmable computer system, such that one of themethods described herein is performed.

Generally, embodiments of the present invention can be implemented as acomputer program product with a program code, the program code beingoperative for performing one of the methods when the computer programproduct runs on a computer. The program code may for example be storedon a machine readable carrier.

Other embodiments comprise the computer program for performing one ofthe methods described herein, stored on a machine readable carrier.

In other words, an embodiment of the inventive method is, therefore, acomputer program having a program code for performing one of the methodsdescribed herein, when the computer program runs on a computer.

A further embodiment of the inventive methods is, therefore, a datacarrier comprising, recorded thereon, the computer program forperforming one of the methods described herein. The data carrier, thedigital storage medium or the recorded medium are typically tangibleand/or non-transitory.

A further embodiment of the inventive method is, therefore, a datastream or a sequence of signals representing the computer program forperforming one of the methods described herein. The data stream or thesequence of signals may for example be configured to be transferred viaa data communication connection, for example via the Internet.

A further embodiment comprises a processing means, for example acomputer, or a programmable logic device, configured to or adapted toperform one of the methods described herein.

A further embodiment comprises a computer having installed thereon thecomputer program for performing one of the methods described herein.

A further embodiment according to the invention comprises an apparatusor a system configured to transfer a computer program for performing oneof the methods described herein to a receiver. The receiver may, forexample, be a computer, a mobile device, a memory device or the like.The apparatus or system may, for example, comprise a file server fortransferring the computer program to the receiver.

In some embodiments, a programmable logic device may be used to performsome or all of the functionalities of the methods described herein. Insome embodiments, a field programmable gate array may cooperate with amicroprocessor in order to perform one of the methods described herein.Generally, the methods may be performed by any hardware apparatus.

The apparatus described herein may be implemented using a hardwareapparatus, or using a computer, or using a combination of a hardwareapparatus and a computer.

The methods described herein may be performed using a hardwareapparatus, or using a computer, or using a combination of a hardwareapparatus and a computer.

While this invention has been described in terms of several embodiments,there are alterations, permutations, and equivalents which will beapparent to others skilled in the art and which fall within the scope ofthis invention. It should also be noted that there are many alternativeways of implementing the methods and compositions of the presentinvention. It is therefore intended that the following appended claimsbe interpreted as including all such alterations, permutations, andequivalents as fall within the true spirit and scope of the presentinvention.

References

[1] ITU-T and ISO/IEC. High efficiency video coding. ITU-TRecommendation H.265|ISO/IEC 23008 10, edition 1, 2013; edition 2, 2014.

[2] J. Chen, E. Alshina, G. J. Sullivan, J.-R. Ohm, J. Boyce, AlgorithmDescription of Joint Exploration Test Model 6, JVET, doc. JVET-F1001,April 2017.

[3] X. Li, H.-C. Chuang, J. Chen, M. Karczewicz, L. Zhang, X. Zhao, A.Said, Multi-Type-Tree, JVET, doc. JVET-D0117, October 2016.

[4] F. Le Léannec, T. Poirier, F. Urban, Asymmetric Coding Units inQTBT, JVET, doc. JVET-D0064, October 2016.

[5] Not yet published European patent application No. 17182209.1

1. An apparatus for encoding a picture, configured to partition thepicture into leaf blocks using recursive multi-tree partitioning,block-based encode the picture into a data stream using the partitioningof the picture into the leaf blocks, wherein the apparatus is configuredto, in partitioning the picture into the leaf blocks, for apredetermined block that corresponds to a predetermined tree level ofthe multi-tree partitioning and which extends beyond a boundary of thepicture, reduce an available set of split modes for splitting thepredetermined block depending on a position at which the boundary of thepicture crosses the predetermined block in order to acquire a reducedset of one or more split modes, wherein, if a cardinality of the reducedset is one, the apparatus is configured to apply the split mode of thereduced set for splitting the predetermined block, and if a cardinalityof the reduced set is greater than one, the apparatus is configured toselect one of the split modes of the reduced set and to apply theselected one of the split modes for splitting the predetermined blockand to signal the selection in the data stream.
 2. The apparatus ofclaim 1, wherein the apparatus is configured to, for reducing theavailable set of split modes to the reduced set of one or more splitmodes, select from the available set of split modes at least one of afirst split mode which does not impose any restriction onto one or moresubsequent splits for subsequent tree levels succeeding thepredetermined tree level, and a second split mode that splits thepredetermined block into two subblocks along a split line that isparallel to the picture boundary's position in the predetermined blockand which split line is at least one of collocated to the boundary ofthe picture and centered inside the predetermined block.
 3. Theapparatus of claim 1, wherein the apparatus is configured to, forreducing the available set of split modes to the reduced set of one ormore split modes, select from the available set of split modes onepredetermined split mode that splits the predetermined block into twosubblocks along a first split line that is parallel to the pictureboundary's position in the predetermined block and which split line isat least one of collocated to the boundary of the picture and centeredinside the predetermined block.
 4. The apparatus of claim 3, wherein thepredetermined split mode is either a vertical bi-split or a horizontalbi-split.
 5. The apparatus of claim 1, wherein the apparatus isconfigured to determine the available set of split modes for thepredetermined block independent from a location of the predeterminedblock relative to the picture boundary, and/or dependent on a sequenceof split modes of preceding tree levels from which the predeterminedblock emerges.
 6. The apparatus of claim 1, wherein the apparatus isconfigured to determine the available set of split modes for thepredetermined block to be equal to a primitive set of split modescomprising a quad split, at least one horizontal bi-split and at leastone vertical bi-split, if all split modes of preceding tree levels fromwhich the predetermined block emerges are outside a restrictive set ofbi-splits, and to be equal to the primitive set of split modes less thequad split, if a split mode of a preceding tree level from which thepredetermined block emerges is one of the restrictive set of bi-splits.7. The apparatus of claim 6, wherein the apparatus is configured to, forreducing the available set of split modes to the reduced set of one ormore split modes, select from the available set of split modes at leastone of the quad split, and a bi-split that splits the predeterminedblock into two subblocks along a split line that is parallel to thepicture boundary's position in the predetermined block and which splitline is at least one of collocated to the picture boundary and centeredinside the predetermined block.
 8. The apparatus of claim 7, wherein, ifa bi-split from the reduced set is applied and the number of consecutivebi-splits is restricted to a predetermined maximum number of consecutivebi-splits, the apparatus is configured to not count a consecutivebi-split if said consecutive bi-split is applied over a pictureboundary.
 9. The apparatus of claim 6, wherein the apparatus isconfigured to, for reducing the available set of split modes to thereduced set of one or more split modes, select from the available set ofsplit modes the quad split only if the predetermined block extendsbeyond a corner of the picture
 12. 10. An apparatus for decoding apicture, configured to partition the picture into leaf blocks usingrecursive multi-tree partitioning, block-based decode the picture from adata stream using the partitioning of the picture into leaf blocks,wherein the apparatus is configured to, in partitioning the picture intothe leaf blocks, for a predetermined block that corresponds to apredetermined tree level of the multi-tree partitioning and whichextends beyond a boundary of the picture, reduce an available set ofsplit modes for splitting the predetermined block depending on aposition at which the picture boundary crosses the predetermined blockto acquire a reduced set of one or more split modes, wherein if acardinality of the reduced set is one, the apparatus is configured toapply the split mode of the reduced set for splitting the predeterminedblock, and if a cardinality of the reduced set is greater than one, theapparatus is configured to select one of the split modes of the reducedset and to apply the selected one of the split modes for splitting thepredetermined block according to a signalization in the data stream. 11.The apparatus of claim 10, wherein the apparatus is configured to, forreducing the available set of split modes to the reduced set of one ormore split modes, select from the available set of split modes at leastone of a first split mode which does not impose any restriction onto oneor more subsequent splits for subsequent tree levels succeeding thepredetermined tree level, and a second split mode that splits thepredetermined block into two subblocks along a split line that isparallel to the picture boundary's position in the predetermined blockand which split line is at least one of collocated to the pictureboundary and centered inside the predetermined block.
 12. The apparatusof claim 10, wherein the apparatus is configured to, for reducing theavailable set of split modes to the reduced set of one or more splitmodes, select from the available set of split modes one predeterminedsplit mode that splits the predetermined block into two subblocks alonga first split line that is parallel to the picture boundary's positionin the predetermined block and which split line is at least one ofcollocated to the boundary of the picture and centered inside thepredetermined block.
 13. The apparatus of claim 12, wherein thepredetermined split mode is either a vertical bi-split or a horizontalbi-split.
 14. The apparatus of claim 10, wherein the apparatus isconfigured to determine the available set of split modes for thepredetermined block independent from a location of the predeterminedblock relative to the picture boundary, and/or dependent on a sequenceof split modes of preceding tree levels from which the predeterminedblock emerges.
 15. The apparatus of claim 10, wherein the apparatus isconfigured to determine the available set of split modes for thepredetermined block to be equal to a primitive set of split modescomprising a quad split, at least one horizontal bi-split and at leastone vertical bi-split, if all split modes of preceding tree levels fromwhich the predetermined block emerges are outside a restrictive set ofbi-splits, and to be equal to the primitive set of split modes less thequad split, if a split mode of a preceding tree level from which thepredetermined block emerges is one of the restrictive set of bi-splits.16. The apparatus of claim 15, wherein the apparatus is configured to,for reducing the available set of split modes to the reduced set of oneor more split modes, select from the available set of split modes atleast one of the quad split, and a bi-split that splits thepredetermined block into two subblocks along a split line that isparallel to the picture boundary's position in the predetermined blockand which split line is at least one of collocated to the pictureboundary and centered inside the predetermined block.
 17. The apparatusof claim 16, wherein, if a bi-split from the reduced set is applied andthe number of consecutive bi-splits is restricted to a predeterminedmaximum number of consecutive bi-splits, the apparatus is configured tonot count a consecutive bi-split if said consecutive bi-split is appliedover a picture boundary.
 18. The apparatus of claim 15, wherein theapparatus is configured to, for reducing the available set of splitmodes to the reduced set of one or more split modes, select from theavailable set of split modes the quad split only if the predeterminedblock extends beyond a corner of the picture.
 19. A method for encodinga picture, comprising: partitioning the picture into leaf blocks usingrecursive multi-tree partitioning, block-based encoding the picture intoa data stream using the partitioning of the picture into the leafblocks, wherein the method further comprises, in partitioning thepicture into the leaf blocks, for a predetermined block that correspondsto a predetermined tree level of the multi-tree partitioning and whichextends beyond a boundary of the picture, reducing an available set ofsplit modes for splitting the predetermined block depending on aposition at which the picture boundary crosses the predetermined blockin order to acquire a reduced set of one or more split modes, wherein ifa cardinality of the reduced set is one, the method comprises applyingthe split mode of the reduced set for splitting the predetermined block,and if a cardinality of the reduced set is greater than one, the methodcomprising selecting one of the split modes of the reduced set andapplying the selected one of the split modes for splitting thepredetermined block and signaling the selection in the data stream. 20.A method for decoding a picture, comprising: partitioning the pictureinto leaf blocks using recursive multi-tree partitioning, block-baseddecoding the picture from a data stream using the partitioning of thepicture into leaf blocks, wherein the method further comprises, inpartitioning the picture into the leaf blocks, for a predeterminedblock, which corresponds to a predetermined tree level of the multi-treepartitioning and which extends beyond a boundary of the picture,reducing an available set of split modes for splitting the predeterminedblock depending on a position at which the picture boundary crosses thepredetermined block in order to acquire a reduced set of one or moresplit modes, wherein if a cardinality of the reduced set is one, themethod comprises applying the split mode of the reduced set forsplitting the predetermined block, and if a cardinality of the reducedset is greater than one, the method further comprising selecting one ofthe split modes of the reduced set and applying the selected one of thesplit modes for splitting the predetermined block according to asignalization in the data stream.
 21. A non-transitory digital storagemedium having stored thereon a computer program for performing a methodfor encoding a picture, comprising: partitioning the picture into leafblocks using recursive multi-tree partitioning, block-based encoding thepicture into a data stream using the partitioning of the picture intothe leaf blocks, wherein the method further comprises, in partitioningthe picture into the leaf blocks, for a predetermined block thatcorresponds to a predetermined tree level of the multi-tree partitioningand which extends beyond a boundary of the picture, reducing anavailable set of split modes for splitting the predetermined blockdepending on a position at which the picture boundary crosses thepredetermined block in order to acquire a reduced set of one or moresplit modes, wherein if a cardinality of the reduced set is one, themethod comprises applying the split mode of the reduced set forsplitting the predetermined block, and if a cardinality of the reducedset is greater than one, the method comprising selecting one of thesplit modes of the reduced set and applying the selected one of thesplit modes for splitting the predetermined block and signaling theselection in the data stream, when said computer program is run by acomputer.
 22. A non-transitory digital storage medium having storedthereon a computer program for performing a method for decoding apicture, comprising: partitioning the picture into leaf blocks usingrecursive multi-tree partitioning, block-based decoding the picture froma data stream using the partitioning of the picture into leaf blocks,wherein the method further comprises, in partitioning the picture intothe leaf blocks, for a predetermined block, which corresponds to apredetermined tree level of the multi-tree partitioning and whichextends beyond a boundary of the picture, reducing an available set ofsplit modes for splitting the predetermined block depending on aposition at which the picture boundary crosses the predetermined blockin order to acquire a reduced set of one or more split modes, wherein ifa cardinality of the reduced set is one, the method further comprisesapplying the split mode of the reduced set for splitting thepredetermined block, and if a cardinality of the reduced set is greaterthan one, the method further comprising selecting one of the split modesof the reduced set and applying the selected one of the split modes forsplitting the predetermined block according to a signalization in thedata stream, when said computer program is run by a computer.
 23. A datastream acquired by the method for encoding a picture according to claim19.
 24. A data stream acquired by the method for decoding a pictureaccording to claim 20.